A standard hammer-drill chuck has a chuck body centered on and rotatable about an axis and formed with a plurality of jaw guides angularly spaced about the axis and all inclined axially forwardly. Respective jaws are provided in the guides, each formed with a row of teeth. An adjustment sleeve generally axially fixed on the body is formed with an adjustment screwthread meshing with the rows of teeth of the jaws so that when the sleeve is rotated on the body in a tightening direction the jaws are displaced axially forward and radially toward one another and when rotated in an opposite loosening direction the jaws are displaced axially backward and radially away from one another. An array of bearing balls is axially engaged between the sleeve and a locking ring rotatable on the chuck body.
According to German patent document 3,411,127 two such ball bearings are provided, axially spaced from each other. The locking ring has a radially projecting flange that engages between the bearings to axially fix it on the chuck body. The balls of the bearings ride in corrugated tracks that increase the clamping pressure when the ball ride up on the ridges. Such a chuck can, however, loosen fairly easily, especially when used in a hammer drill.